The essence of applying coatings by gas detonation rests in the following. A barrel open at one end is filled with an explosive gaseous mixture, a powder for obtaining a coating is fed, and detonation is initiated at the closed end of the barrel. The high-temperature (.about.4000.degree. C.) and high-speed (.about.1500 m/s) flow of detonation products heats and accelerates the powder particles which are thrown against the surface of a workpiece before the open end of the barrel to form a coating. Normally, the apparatus for applying coatings by gas detonation are automatically controlled, the detonation process being repeated at a frequency of about 10 shots per second.
One of the major units of the gas detonation apparatus is an arrangement for conveying a powder material to the barrel. This arrangement is intended to convey a preset quantity of powder to a preset section of the barrel at a preset point in time. The more accurately such conditions are conformed to the higher is the quality of coatings, expressed by such characteristics as stability of the coating, strong bond between the coating and the surface of the workpiece, porosity, and thickness of the coating after one shot.
Among factors complicating normal functioning of such arrangements in apparatuses for applying coatings by gas detonation are the following: penetration of the products of detonation from the barrel to the housing of the powder-feeding arrangement (backfire) and penetration of the explosive mixture to the housing of the arrangement as this mixture is fed to the barrel of the apparatus. These two factors can lead to failure of the apparatus.
Therefore, the arrangement for conveying powder to the barrel should be so constructed as ensure an extra service life, stable cyclical feeding of the powder with particle size of 5 to 50 mkm (at a frequency of 1 to 10 cycles per second), an accurately preset quantity of powder for each cycle, reliable protection of the arrangement against backfires, uniform distribution of the powder particles in the flow of detonation products, fluidity of the powder as it fills the powder-metering recess, amenability to automatic remote control over the quantities of powder and carrier gas, and provision for minimizing the quantity of powder-carrying gas entering the barrel.
At present, apparatuses for applying coatings by gas detonation use two types of powder feeding arrangements, particularly pneumatic and mechanical. In the pneumatic arrangements powder is conveyed to the barrel of the apparatus by a compressed gas continuously or in a pulse-wise manner. In the continuous feeding of the powder to the barrel the quantity of powder is spread in the barrel in the form of an elongated mist. In this case the difference in the initial position of the powder particles axially of the barrel leads to variations in the speed and heating temperature of the particles as they are accelerated and heated by detonation products in the barrel of the apparatus. Experiments have shown that for obtaining high-quality coatings from a preselected powder material it is necessary that during the escape of the powder particles from the barrel the speed and temperature of all the particles of the quantity of powder fed to the barrel be approximately equal. This condition cannot be complied with during continuous feeding of the powder to the barrel, and therefore continuous feeding of the powder to the barrel results in low quality coatings with unstable characteristics.
During pulsewise feeding of the powder to the barrel in agreement with the working cycle of the apparatus the metered quantity of powder is localized in a small volume in the zone where the powder is introduced to the barrel, whereby a negligeable spread in the speed and temperature of powder particles at the outlet from the barrel and near the surface being coated is ensured. The pulsewise feeding of powder to the barrel allows to control the process of coating application.
The quantity of powder fed to the barrel is preset in various manners, such as by the diameter of pipe carrying the powder, or by the depth to which a rod with a powder-metering recess is immersed in a hopper filled with the powder. An inherent disadvantage of such powder metering arrangement rests in variations (to tens of percent) in the quantity of powder introduced to the barrel because of low fluidity of the powder or a change in the volume of the powder metering device due to abrasive wear of the metering recess eventually resulting in unstanble characteristics of coatings.
The known powder feeding arrangements lack means for protecting against backfires propagating through the powder-conveying gas pipes. In order to prevent the action of backfires, the pipes carrying the powder are elongated which leads to delays in power supply, i.e., a slower response of the arrangement.
An arrangement bearing the closest resemblance to one to be described in the present invention includes a pressure-sealed cylindrical housing accomodating a piston valve driven by compressed gas.
The housing comprises a hopper containing a powder and communicating with the interior of the housing, and a slide valve provided with a powder-metering recess and a drive, the housing having through passages for conveying the powder from the hopper to the powder-metering recess, for feeding a compressed gas and for conveying the powder from the powder-metering recess to the barrel. The powder-metering recess is provided in the slide valve and arranged so that in one of its extreme positions it communicates with the hopper where it is charged with the powder, whereas in the other extreme position it communicates with holes for feeding the powder to the barrel and for feeding the compressed gas carrying the powder from the powder-metering recess to the barrel. In this construction of the arrangement the quantity of powder fed to the barrel is preset by the volume of the powder-metering recess, whereas the piston valve protects the powder-metering device against backfires, as the hole through which the powder is fed to the barrel is closed by the piston valve during detonation.
However, with this construction of the powder-metering device the clearance between the containing surface of the piston valve and housing is susceptible to penetration therein of the powder fed to the barrel to result in possible jamming of the slide valve and failure of the powder-metering device. Also, penetration of the powder to the clearance between the slide valve and housing results in fast wear of contacting surfaces and reduces the quantity of powder fed to the barrel. For this reason, losses of powder can be as high as 30-40% of the powder quantity determined by the powder-metering recess resulting in unstable thickness of the coating applied by one shot.